Where are you located? The Shoppers Food Warehouse down by me in Alexandria (on Rt 1) always seems to have a lot of ready-made cupcakes and other baked goods available, although I can't recall if they have boxes of all vanilla.
You might be able to ask staff at a grocery store (especially one like Wegmans) to just make you a box on-the-spot from cupcakes they may have in the back, actually. Or maybe just resign yourself to getting a few mixed boxes to ensure you get the number of vanilla ones you need. Cupcakes freeze well for later eating if needed.
